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| United States Patent |
5,045,162
|
|
Herbst
, et al.
|
September 3, 1991
|
Process for electrochemically regenerating chromosulfuric acid
Abstract
Chromosulfuric acid, which is used in many organic reactions as an
oxidizing agent, can advantageously be regenerated electrochemically if an
electrolysis cell is used which comprises two tub-like half shells with a
current-permeable, hydraulically sealing partition situated inbetween. The
hydrogen produced at the cathode can also be extracted and utilized in
this way.
| Inventors:
|
Herbst; Hans (Meitingen, DE);
Stenzel; Jurgen (Gersthofen, DE);
Benninger; Siegfried (Burgkirchen, DE)
|
| Assignee:
|
Hoechst Aktiengesellschaft (Frankfurt, DE)
|
| Appl. No.:
|
606437 |
| Filed:
|
October 31, 1990 |
Foreign Application Priority Data
| Current U.S. Class: |
205/770 |
| Intern'l Class: |
C02F 001/46 |
| Field of Search: |
204/130,151
|
References Cited
U.S. Patent Documents
| 3761369 | Sep., 1973 | Tirrell | 204/130.
|
Other References
Bergner, D. et al, Hoechst-Uhde-Membranzellen: Oct. (1984) (Jahrestagung
der Fachgruppe Angew. Elektrochem. of the German Society of Chemists).
|
Primary Examiner: Tufariello; T. M.
Claims
We claim:
1. A process for electrochemically regenerating chromosulfuric acid by
anodic oxidation of Cr.sup.3+ ions to Cr.sup.6+ ions, in which the anolyte
contains 20 to 200 g/dm.sup.3 total CrO.sub.3 and 100 to 600 g/dm.sup.3
H.sub.2 SO.sub.4 and the catholyte contains 50 to 500 g/dm.sup.3 H.sub.2
SO.sub.4, which process comprises carrying out the regeneration in an
electrolysis cell which comprises two tub-like metal half shells whose
open sides face one another, a metal plate which is provided with holes or
slots and which is joined to the anode tub by corrugated fasteners being
present in the anode tub as anode, a metal sheet which is joined to the
cathode tub by corrugated fasteners being present in the cathode tub as
cathode, anode tub and cathode tub being separated from one another by a
current-permeable, hydraulically sealing partition and seals and being
held together by a clamping device, the temperature being 40.degree. to
110.degree. C. and the current density 100 to 2500 A/m.sup.2.
2. The process as claimed in claim 1, wherein an anode is used which is a
titanium, zirconium, vanadium or tantalum expanded metal.
3. The process as claimed in claim 2, wherein the anode is activated by a
coating which has a high overvoltage with respect to oxygen.
4. The process as claimed in claim 1, wherein a cathode is used which is a
slatted metal sheet made of nickel or a nickel alloy.
5. The process as claimed in claim 1, wherein an ion exchanger membrane
which is composed of a perfluorinated polymer is used as
current-permeable, hydraulically sealing partition.
6. The process as claimed in claim 1, wherein the electrolysis is carried
out in a bipolar cell.
Description
DESCRIPTION
The invention relates to a process for electrochemically regenerating
chromosulfuric acid in which a novel electrolysis cell is used.
In the electrochemical regeneration of chromosulfuric acid which has been
used in oxidation processes, electrodes made of lead or lead alloys and
electrolysis cells having lead walls, for example steel troughs lined with
lead, are normally used.
The disadvantage of using lead or lead alloys is that the anodes lose their
activity relatively fast and can be reutilized only to a limited extent
and that a high hydrogen overvoltage (approximately 1.1 volt) appears at
cathodes made of lead and lead alloys. It has also not yet been possible
to carry out the electrolysis in sealed cells and to extract, in addition
to the Cr.sup.6+, also the hydrogen evolved at the cathode side. On the
contrary, the hydrogen produced has to be sucked off the cells covered
with foils and diluted with air in a ratio of about 50:1 so that work can
be carried out safely below the explosion limit of about 4% H.sub.2 in
air. This procedure is uneconomical and open to objection for reasons of
occupational hygiene and environmental protection.
A membrane cell for alkali-metal chloride electrolysis which comprises two
half shells, one half shell being pressed from titanium sheet and the
other from stainless steel or nickel sheet, is known (cf. Bergner and
Hannesen, GDCH-Jahrestagung Angew. Elektrochemie, October 1984). The
electrodes are each composed of a slatted metal sheet which is activated
and welded into the half shell. At regular intervals, the electrodes are
joined to the rear walls of the half shells by corrugated fasteners. The
two half shells of a cell are separated from one another by a permeable
membrane and seals.
The object was to find a process in which the chromosulfuric acid is
regenerated in a sealed cell system and the hydrogen produced can be
extracted.
It was found that the membrane cell developed for alkali-metal chloride
electrolysis is also suitable in principle for the electrochemical
regeneration of chromosulfuric acid.
The invention consequently relates to a process for electrochemically
regenerating chromosulfuric acid by anodic oxidation of Cr.sup.3+ ions to
Cr.sup.6+ ions, in which the anolyte contains 20 to 200 g/dm.sup.3 total
CrO.sub.3 and 100 to 600 g/dm.sup.3 H.sub.2 SO.sub.4 and the catholyte
contains 50 to 500 g/dm.sup.3 H.sub.2 SO.sub.4, which process comprises
carrying out the regeneration in an electrolysis cell which comprises two
tub-like metal half shells whose open sides face one another, a metal
plate which is provided with holes or slots and which is joined to the
anode tub by corrugated fasteners being present in the anode tub as anode,
a metal sheet which is joined to the cathode tub by corrugated fasteners
being present in the cathode tub as cathode, anode tub and cathode tub
being separated from one another by a current-permeable, hydraulically
sealing partition and seals and being held together by a clamping device,
the temperature being 40.degree. to 110.degree. C. and the current density
100 to 2500 A/m.sup.2.
The electrolysis cell to be used for the process according to the invention
is explained with reference to FIGS. 1 to 4.
FIG. 1 shows a perspective overall view of an electrolysis cell,
FIG. 2 shows a section along the line II--II in FIGS. 1, 3 and 4,
FIG. 3 shows a section along the line III--III in FIGS. 1, 2 and 4 and
FIG. 4 shows a plan view in the direction of the arrow IV in FIGS. 1, 2 and
3.
According to FIG. 1, the cell comprises two tub-like metal half shells (1)
and (2). The anode tub (1) contains a perforated or slotted plate (3)
(perforated metal sheet, expanded metal or the like) which is joined to
the anode tub (1) by means of corrugated fasteners (4). The plate (3) acts
as anode. The cathode tub (2) contains a metal sheet (5) as cathode which
is connected to the tub (2) by means of corrugated fasteners (6). The
cathode is composed of a simple metal sheet, metal sheet strips,
perforated metal sheet, expanded metal or slatted metal sheet, preferably
of a slatted metal sheet.
Anode tub (1) and cathode tub (2) are separated from one another by a
current-permeable, hydraulically sealing partition (7) and seals (8) and
(9). They are held together to form a unit by two steel frames (10) and
(11) which are screwed to each other in an insulated manner. The screws
(16) are insulated by means of plastic bushes (17) and plastic washers
(18). Located at the bottom of the anode tub (1) is an inlet pipe (12) for
the anolyte and located on the cathode tub (2) is an inlet pipe (14) for
the catholyte. Located at the top of the tubs (1) and (2) are the drainage
pipes (13) and (15).
FIG. 2 shows, in addition, the position of the corrugated fasteners (4) and
(6) and also the offset mounting of the inlet pipes (12) and (14).
From FIGS. 3 and 4 finally the encircling steel frame (11) can be seen.
The anode tub (1) and the corrugated fasteners (4) are composed of
titanium, whereas the cathode tub (2) and the corrugated fasteners (6) are
composed of nickel or a nickel alloy, for example .RTM. Hastelloy.
It was found that the so-called "valve metals" titanium, tantalum, vanadium
and zirconium used already in alkali-metal chloride electrolysis are also
suitable as materials for the anode (3) under the corrosive conditions of
chromic acid electrolysis. Under anodic current loading, these metals form
a coherent oxide film on their surface which protects the basic material.
If the surface of the anodes is not activated, the oxide layer formed
prevents further current flow. For chromic acid electrolysis, only
electron-conducting oxides which exhibit a high overvoltage with respect
to oxygen, for example lead dioxide, manganese dioxide, tin dioxide,
tantalum oxides or iridium oxides, are possible as suitable activation
layers for these metals. One of the highest overvoltages for oxygen is
exhibited by lead dioxide, which is preferred. Thus, the electrochemical
reaction
Cr.sup.3+ +4H.sub.2 O-3e.sup.- .fwdarw.CrO.sub.4.sup.2- +8H.sup.+
which proceeds anodically, yields current efficiencies of between 96% and
88% with current densities of 200 A/m.sup.2 to 2500 A/m.sup.2 at a
titanium anode coated with PbO.sub.2.
Suitable materials for the cathode (5) are nickel and nickel alloys, for
example Hastelloy. If sulfuric acid is used as catholyte, protons are
discharged and hydrogen evolved, which leaves the cathode space as a gas,
at the cathode in accordance with the reaction equation
3H.sup.+ +3e.sup.- .fwdarw.1.5H.sub.2
Nickel is only resistant to 10 to 35% sulfuric acid, however, if it is
cathodically polarized. It is therefore necessary to prevent the nickel
cathodes being exposed at zero current to the sulfuric acid.
At a current density of 100 A/m.sup.2, the hydrogen overvoltage at nickel
is relatively low at a level of 0.42 V, and in comparison therewith it is
fairly high at a level of 1.09 V at lead under the same conditions. This
has the consequence that the use of nickel as cathode results in a
correspondingly lower cell voltage.
Cation-active ion exchanger membranes made of perfluorinated polymers
containing sulfonyl groups have proved very satisfactory as
current-permeable, hydraulically sealing partition (7) between anode space
and cathode space. They exhibit an excellent durability and selectivity in
the electrolytes used up to temperatures of 110.degree. C. The use of such
membranes makes it possible to collect the cathodically evolved hydrogen
separately and supply it to a further utilization.
The electrolysis cell to be used and assembled according to the invention
can be operated after filling the cathode space with catholyte and the
anode space with anolyte and after pressing current leads from a rectifier
onto the anode tub rear wall and cathode tub rear wall. Catholyte and
anolyte are each continuously fed in from stock containers by means of
pumps at the lower end of the electrode space. The electrolyte leaves the
cell at the top end. The anolyte with the desired composition is supplied
for further use, while the catholyte is continuously circulated via a
buffer container and concentrated again from time to time.
The oxygen evolution (due to water decomposition) which proceeds to a small
extent at the anode and is undesirable per se ensures an adequate mixing
of the anolyte and promotes the diffusion of Cr.sup.3+ at the anode
surface. This effect can be intensified by additionally injecting inert
gas into the anode space.
Preferably, this electrolysis cell is not operated separately. On the
contrary, in a cell assembly a plurality of cells is pressed together rear
wall to rear wall by means of a clamping device. Consequently, the current
fed in with copper rails at the beginning of the cell array is able to
flow through all the cells and is drained at the end of the array by
copper rails. Special contact strips ensure a good current transfer
between the cells. If the cells are operated in this manner, the cell is a
bipolar one. All the individual elements are connected in series.
The concentration in the anolyte is 20 to 200, preferably 100 to 200, in
particular 130 to 170 g/dm.sup.3 total CrO.sub.3 and 100 to 600,
preferably 300 to 600, in particular 450 to 550 g/cm.sup.3 H.sub.2
SO.sub.4. The catholyte contains 50 to 500, preferably 300 to 350
g/dm.sup.3 H.sub.2 SO.sub.4.
The electrolysis is carried out at a temperature of 40.degree. to
110.degree., preferably 80.degree. to 110.degree. C., and at a current
density of 100 to 2500, preferably 500 to 2500 A/m.sup.2.
The process according to the invention will now be explained by way of the
following examples.
EXAMPLE 1
The electrolysis was carried out in a round laboratory membrane cell which
was composed of 2 glass shells and was flanged together so as to seal by
means of two PTFE O-rings. The two glass shells formed the cathode space
and anode space. They were separated by a polymer membrane made of a
perfluorinated polymer which was clamped between the two O-rings.
The two circular electrodes were eccentrically mounted and direct current
was supplied via these mountings. It was possible to vary anode and
cathode in their distance from one another and from the membrane by means
of spacing strips. Anolyte and catholyte were heated with heating rods to
90.degree. C. in the two cell halves and were kept constant at this
temperature during the electrolysis.
______________________________________
Anode space volume:
95 cm.sup.3
Cathode space volume:
90 cm.sup.3
Active anode area: 36 cm.sup.2
Cathode area: 36 cm.sup.2
______________________________________
The cathode was composed of non-activated nickel expanded metal, the anode
of titanium expanded metal which was coated on all sides with
electrodeposited PbO.sub.2. The cathode-anode spacing was 8 mm.
______________________________________
Anolyte: 550-560 g/l
H.sub.2 SO.sub.4
200 g/l total CrO.sub.3 104 g/l Cr
Catholyte: 440-445 g/l
H.sub.2 SO.sub.4 (35%)
______________________________________
At the same time, the catholyte was circulated by pumping through the
cathode space at an throughput of 9 cm.sup.3 /h which was constant for all
current densities. The electroylsis are data obtained shown in Table 1.
TABLE 1
______________________________________
Current density (A/m.sup.2)
500 1,500 2,500
Throughput (cm.sup.3 /h)
21 60 94
Degree of oxidation (%)
50 52 52
Cell voltage (V)
2.25 2.56 2.90
Current efficiency based
96.0 91.5 88.5
on Cr.sup.6+ formation (%)
Energy requirement
1.95 2.40 2.90
(kWh/kg CrO.sub.3)
Total running time:
249 days
Total current consumption:
26,000 Ah 7,429 kAh/m.sup.2
CrO.sub.3 produced:
28.97 kg 8,277 kg/m.sup.2
______________________________________
EXAMPLE 2
A titanium expanded metal anode activated with tantalum oxide/iridium oxide
mixture was tested for its suitability in a second glass cell which
corresponded completely to the cell described above in its construction.
______________________________________
Cathode: nickel expanded metal
Anode: titanium expanded metal activated with
Ta.sub.2 O.sub.5 /IrO.sub.2
Membrane: perfluorinated polymer
Temperature: 90.degree. C.
Anode-cathode spacing:
8 mm
Electrolyte composition
as in Example 1
______________________________________
A notable feature was the low cell voltage of 1.92 V for a current loading
of 500 A/m.sup.2 compared with that in Example 1. However, the current
efficiency in relation to CrO.sub.3 formation of on average only 61% with
a comparatively low degree of oxidation of 44% was lower. This resulted in
a relatively high energy requirement of 2.65 kWh/kg CrO.sub.3 at 500
A/m.sup.2. A heavier gas evolution (analyzed as O.sub.2), which is
attributable to the lower oxygen overvoltage of this activation coating
compared with PbO.sub.2, was to be observed on the anode side. With
increasing current loading, the current efficiency decreased still further
and reached only about 49%, for example, at 1,500 A/m.sup.2.
EXAMPLE 3
Chromosulfuric acid was electrolytically regenerated in a membrane cell as
described in FIGS. 1 to 4.
______________________________________
Anode space volume:
1,150 cm.sup.3
Cathode space volume:
870 cm.sup.3
Anode area: 285 cm.sup.2
Cathode area: 285 cm.sup.2
Anode material: titanium expanded metal
activated with PbO.sub.2
Cathode material:
slat-type nickel lamella sheet
Cathode-anode spacing:
9 mm
Anolyte: 470 g/l H.sub.2 SO.sub.4
160 g/l total CrO.sub.3 83.2 g/l Cr
Catholyte: 440-445 g/1 H.sub.2 SO.sub.4
Temperature: 85-95.degree. C.
Current density:
500 A/m.sup.2
Throughput: 140 cm.sup.3 /h
Degree of oxidation:
65%
Cell voltage: 2.65 V
Current efficiency:
91% based on Cr.sup.6+ formation
Energy requirement:
2.25 kWh/kg CrO.sub.3
______________________________________
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